Two-way radio communication systems supporting dispatch applications provide both radio-to-radio and dispatcher-to-radio communication capabilities. Since the priority of communications in such an application is frequently associated with the source of a message, these radio systems provide a mechanism to allow certain messages to have priority access to the communications systems, even preempting currently active messages to provide it.
FIG. 1 illustrates, in block diagram form, a communications system, generally depicted by the numeral 100. A control console 101, including one or more operator positions, is shown coupled to a digital interface unit or DIU 102 and then coupled to a repeater station 103 via a point-to-point interface 107. The point-to-point interface 107 may be telephone lines, dedicated hard-wired lines, a microwave link, etc. The console 101 may also be connected to other repeater stations via DIUs, base stations, and satellite receivers as well-known in the art. The communication system as shown supports communications among a number of mobile or portable communication units 104, 105 and 106.
In normal operation, signals from a communication unit 104 are transmitted to the repeater station 103. The repeater station 103 then sends the received signals to the console 101 by way of the DIU 102. It also retransmits the received signal to the other communication units 105 and 106. If, during this operation, it is necessary for the console operator to communicate with the mobile units with a higher priority message, the console operator will initiate a transmission from the console 101. The DIU 102 then converts the console message from its analog form to a digital form, compatible with the communication system. When the console message reaches the repeater station 103, the station must switch from retransmitting the received signal to transmitting the console signal.
The current state of the art, for analog communications systems, performs the above switching function with timing that is unrelated to the content of the two messages. The repeated message is terminated and the console message substituted as rapidly as the repeater station is able to make the change.
State of the art digital communication systems present difficulties when attempting to use these old techniques. The frame-synchronous nature of most advanced digital voice signaling formats results in significant disruption to system operation if switching is performed in an uncoordinated manner. Among other reasons, this disruption can be attributed to the necessity for reacquiring system timing and synchronization information for the new message. Accordingly, a technique to accomplish the switching functions in an expeditious manner for digital communication systems is needed.